MORPHOLOGY OF EYE MUSCLE NERVES 83 



jacent mesenchymatous cells between which they lie. In cross 

 sections of the nerve anlage (fig. 52, p. 181) the fibers appear as 

 black dots or granules scattered in a granular and vacuolated pro- 

 toplasmic envelope. Their appearance is essentially the same in 

 VomRath preparations as in preparations by the Cajal or the 

 Bielchowsky-Paton methods. The identity of this bundle of fi- 

 brils with the trochlearis in advanced stages of differentiation is 

 established, not only by its fibrillar structure, but by its relations 

 with tube and myotome at this and all subsequent stages of de- 

 velopment. As this anlage has been traced in unbroken contin- 

 uity until the differentiation corresponds essentially to that of 

 the adult nerve there is no good reason to doubt the identity of 

 the fibers of the anlage with those of the adult and that the neu- 

 rofibrillae make their appearance within them. It is true that 

 the splitting of the fibers of the nerve anlage into the neurofibrils 

 of the fully differentiated nerve is a matter of inference rather 

 than one of direct observation, yet students of nerve histogenesis 

 have not hesitated to make this inference which has been drawn 

 by Bardeen ('03), Carpenter ('06) and Paton ('07), on the 

 ground that, while the primitive fibrils of the nerve anlage are 

 relatively coarse structures, the neurofibrillae of later stages are 

 much finer and more numerous. There is no reason for inferring 

 that the neurofibrils have any other origin than the coarser fibrils 

 of the nerve anlage, which make their appearance in the nerve 

 from its first inception in the stages when the nerve anlage is 

 wholly free from cells. Furthermore, at no time in the histogene- 

 sis of the nerve is there any indication of a genetic relation of the 

 cells, which later make their appearance in the nerve, to the fibers. 

 No neuroblasts have been discovered within the nerve anlage. So 

 that there seems to be no reason why the histogenesis of the neu- 

 rofibrils of the trochlearis may not be regarded as in every respect 

 similar to that of the neurofibrils of spinal somatic motor nerves. 

 In other words they are to be regarded as the product of differen- 

 tiation within the neuraxon processes of medullary neuroblasts 

 situated in the somatic motor column of the neural tube. 



Miss Piatt ('91a, p. 96) inferred the neuroblastic origin of the 

 fibers of the proximal portion of the trochlearis on the basis of 



